A retroreflective sheet for reflecting incoming light toward a light source has been well known so far and the sheet using its retroreflective characteristic is widely used in the above fields. Particularly, a cube-corner retroreflective sheet using the retroreflective theory of a cube-corner retroreflective element such as a triangular-pyramidal reflective element is extremely superior to a conventional retroreflective sheet using a micro glass beads in retroreflectivity and its purpose has been expanded year by year because of its superior retroreflective performance.
However, though a conventionally-publicly-known triangular-pyramidal retroreflective element shows a preferable retroreflectivity when the angle formed between the optical axis of the element (axis passing through the apex of the triangular pyramid of the triangular-pyramidal retroreflective element equally separate from three faces constituting a triangular-pyramidal cube-corner retroreflective element and intersecting each other at an angle of 90.degree.) and an incident light (the angle is hereafter referred to as entrance angle) is kept in a small range, the retroreflectivity rapidly deteriorates as the entrance angle increases (that is, the entrance angle characteristic deteriorates). Moreover, a light entering the triangular-pyramidal retroreflective element face at an angle less than a critical angle (.alpha..sub.c) satisfying an internal total-reflection condition determined by the ratio between the refractive index of a transparent medium constituting the triangular-pyramidal retroreflective element and the refractive index of air penetrates into the back of the element without totally reflecting on the interface of the element. Therefore, a retroreflective sheet using a triangular-pyramidal reflective element generally has a disadvantage that it is inferior in entrance angularity.
However, because a triangular-pyramidal retroreflective element can reflect light in the light incoming direction almost over the entire surface of the element, reflected light does not reflect by dispersing at a wide angle due to aberration differently from the case of a micro-glass-ball reflective element. However, the narrow dispersion angle of the reflected light practically easily causes a trouble that, when the light emitted from a head lamp of an automobile is retro-reflected on a traffic sign, the retro-reflected light hardly reaches, for example, a driver present at a position distant from the axis of the retro-reflected light. Particularly when the distance between an automobile and a traffic sign decreases, the above trouble more-frequently occurs because the angle formed (observation angle) between the entrance axis of a light ray and the axis (observation axis) connecting a driver and a reflective point increases (that is, the observation angularity deteriorates).
For the above cube-corner retroreflective sheet, particularly for a triangular-pyramidal cube-corner retroreflective sheet, many proposals have been known so far and various improvements and studies are performed.
For example, Jungersen's U.S. Pat. No. 2,481,757 discloses a retroreflective sheet constituted by arranging retroreflective elements of various shapes on a thin sheet and a method for manufacturing the sheet. Triangular-pyramidal reflective elements disclosed in the above U.S. patent include a triangular-pyramidal reflective element in which the apex is located at the center of a base-plane triangle and the optical axis does not tilt and a triangular-pyramidal reflective element in which the apex is not located at the center of a base-plane triangle but the optical axis tilts. Moreover, it is described in the U.S. patent to efficiently reflect light toward an approaching automobile. Furthermore, it is described that the size of a triangular-pyramidal reflective element, that is, the depth of the element is 1/10 in (2,540 .mu.m) or less. Furthermore, FIG. 15 in the U.S. patent illustrates a triangular-pyramidal reflective element whose optical axis tilts in the plus (+) direction similarly to the case of a preferred mode of the present invention. The tilt angle (.theta.) of the optical axis is estimated as approx. 6.5.degree. when obtaining it from the ratio between the longer edge and shorter edge of the base-triangular plane of the illustrated triangular-pyramidal reflective element.
However, the above Jungersen's U.S. patent does not specifically disclose a very small triangular-pyramidal reflective element shown in the present invention or it does not disclose a size or an optical-axis tilt a triangular-pyramidal reflective element must have in order to show superior observation angularity and entrance angularity.
In this specification, the expression "optical axis tilts in the plus (+) direction" represents, as described later, that the optical axis tilts in the direction in which the difference (q-p) between the distance (q) from the intersection (Q) between the optical axis of the triangular-pyramidal reflective element and the base plane (X-X') of the triangular-pyramidal reflective element up to the base edges (x, x, . . . ) shared by the element pair {the distance (q) is equal to the distance from the intersection (Q) up to a plane (Y-Y') vertical to the base plane (X-X') including the base edges (x, x, . . . ) shared by the element pair} and the distance (p) from the intersection (P) between a perpendicular extended from the apex of the element to the base plane (X-X') and the base plane (X-X') up to the base edges (x, x, . . . ) shared by the element pair becomes plus (+). However, when the optical axis tilts in the direction in which (q-p) becomes minus (-), the expression "optical axis tilts in the minus (-) direction" is hereafter used.
Moreover, Stamm's UP Pat. No. 3,712,706 discloses a retroreflective sheet in which so-called equilateral triangular-pyramidal cube-corner retroreflective elements whose base-plane triangles are equilateral triangles are arranged on a thin sheet so that their base planes are brought into a close-packed state on a common plane. Stamm's U.S. patent improves the problem that a retroreflectivity is deteriorated due to increase of an entrance angle through mirror-reflection by vacuum-coating the reflective surface of a reflective element with a metal such as aluminum and the above trouble that the light incoming at an angle of less than an internal total reflection condition passes through the interface between elements and thereby, it does not retro-reflect.
However, because the above Stamm's proposal uses the mirror reflection theory as means for improving the angularity (wide angularity), the proposal easily causes the trouble that the appearance of an obtained retroreflective sheet becomes dark or the reflection brightness easily deteriorates because a metal such as aluminum or silver used for the mirror surface is oxidized due to incoming of water or air while it is used. Moreover, the proposal does not describe means for improving the angularity (wide angularity) by a tilt of an optical axis at all.
Moreover, Hoopman's European Pat. No. 137,736B1 describes a retroreflective sheet in which triangular-pyramidal cube-corner retroreflective elements with a tilted optical axis whose triangular base-plane are isosceles triangles are arranged on a thin sheet so that their base planes are brought into a close-packed state on a common plane. The optical axis of a triangular-pyramidal cube-corner retroreflective element described in the patent tilts in the minus (-) direction inversely to the tilt direction of the optical axis of a preferred triangular-pyramidal reflective element of the present invention and it is described in the patent that the tilt angle of the optical axis is approx. 7.degree. to 13.degree..
Furthermore, Szczech's U.S. Pat. No. 5,138,488 discloses a retroreflective sheet in which triangular-pyramidal cube-corner retroreflective elements with a tilted optical axis whose base-plane triangles are isosceles triangles are arranged so that the base planes is brought into a close-packed state on a common plane. In the case of the U.S. patent, it is specified that the optical axis of each of the above triangular-pyramidal retroreflective elements tilts in the direction of a side shared by a pair of retroreflective elements facing each other, the tilt angle of the optical axis ranges between 2.degree. and 5.degree., and the size of each element ranges between 25 .mu.m and 100 .mu.m.
Furthermore, in the case of European Pat. No. 548,280B1 corresponding to the above patent, it is described that the direction of a tilt of an optical axis includes a side common to a pair of elements, the distance between a plane vertical to a common plane and the apex of an element is not equal to the distance between the point where the optical axis of an element intersects the common plane and the vertical plane, the tilt angle of the optical axis of the element ranges between 2.degree. and 5.degree., and the size of the element ranges between 25 .mu.m and 100 .mu.m.
As described above, in the case of Szczech's European Pat. No. 548,280B1, the tilt of an optical axis ranges between +2.degree. and +5.degree. and between -2.degree. and -5.degree.. In the case of embodiments of the above Szczech's U.S. patent and European patent, however, only triangular-pyramidal retroreflective elements are specifically disclosed which have optical-axis tilt angles of -8.2.degree., -9.2.degree., and -4.3.degree. and an element height (h) of 87.5 .mu.m.
The above-described conventionally publicly-known triangular-pyramidal cube-corner retroreflective elements of Jungersen's U.S. Pat. No. 2,481,757. Stamm's U.S. Pat. No. 3,712,706, Hoopman's European Pat. No. 137,736B1 and Szczech's U.S. Pat. No. 5,138,488 and European Pat. No. 548,280B1 are common in that the base planes of a plurality of triangular-pyramidal reflective elements serving as cores of entrance and reflection of light are present on the same plane. Every retroreflective sheet constituted with triangular-pyramidal reflective elements whose base planes are present on the same plane is inferior in entrance angularity, that is, every retroreflective sheet has a disadvantage that retroreflective brightness rapidly decreases when the entrance angle of a light to the triangular-pyramidal reflective elements increases.
In general, high brightness, that is, the height (magnitude) and angularity (wide angularity) of a reflection brightness represented by the reflection brightness of the light incoming from the front of a triangular-pyramidal cube-corner retroreflective sheet are required for the sheet as optical characteristics. Moreover, three performances such as observation angularity, entrance angularity, and rotational angularity are requested for the angularity (wide angularity) of the retroreflective sheet.
As described above, every retroreflective sheet constituted with conventionally publicly-known triangular-pyramidal cube-corner retroreflective elements has a low entrance angularity and has no satisfied observation angularity in general. However, the present inventor et al. found through a ray tracing simulation that it is possible to improve the entrance angularity of the retroreflective sheet constituted with the triangular-pyramidal reflective elements by making the depth (h') of a plane (plane c) having one base edge on the base plane (X-X') of the triangular-pyramidal reflective elements from the apexes (H.sub.1, H.sub.2) of the elements {the depth (h') is equal to the height of the apexes (H.sub.1, H.sub.2) from the base plane (X-X')} substantially larger than the depth (h) of a plane (virtual plane Z-Z') including the base edges (z, w) of two planes (plane a, plane b) substantially perpendicularly intersecting the planes c of the triangular-pyramidal reflective elements.